LIN-35 is necessary in both the soma and germline for preserving fertility in Caenorhabditis elegans under moderate temperature stress

Maintenance of germline function under stress conditions is crucial for species survival. The germ line in many species is especially sensitive to elevated temperature. We have investigated the role of the pocket protein LIN-35 in preserving fertility in Caenorhabditis elegans under moderate temperature stress. We show that lin-35 mutants display several temperature sensitive germline defects, and more severe reductions in brood size at elevated temperatures compared to wild type. This loss of fertility under temperature stress is primarily due to loss of zygotic, but not maternal, LIN-35. Additionally, we have found that expression of LIN-35 is necessary in both the germ line and soma for the preserving fertility under moderate temperature stress. Specifically, while LIN-35 function in the germ line is required for maintaining fertility in hermaphrodites, broad somatic expression of LIN-35 is also necessary for oocyte formation and/or function under moderate temperature stress. Together, our data add to the emerging understanding of the critical role that LIN-35 plays in preserving tissues against stress.


Introduction
Pocket proteins generally function in transcriptional repression, which in mammals has been linked to cell cycle regulation and cell fate decisions [1,2]. In C. elegans, the sole pocket protein is LIN-35, which is homologous to the three mammalian family members Rb (retinoblastoma), p107, and p130 [3]. Although LIN-35 is expressed broadly across most tissue types, it has distinct roles in different tissues including the repression of germline gene expression and RNAi pathways in the soma, suppression of nuclear divisions in the intestine, and regulation of apoptosis in the germ line [4][5][6][7]. C. elegans carrying mutations in lin-35 also demonstrate a range of synthetic phenotypes (those that appear only when a second gene is also disrupted), which have revealed roles for LIN-35 in regulating vulval cell fate specification, somatic apoptosis, and somatic gonad development [8][9][10]. Despite the role of LIN-35 in these varied processes, lin-35 mutants remain viable and fertile at normal culture temperatures. However, they do display slowed growth rates and brood sizes [5,11]. C. elegans mutant for lin- 35 show substantial changes in gene expression in the germ line, suggesting that LIN-35 has an intrinsic function in germ cells [12]. However, lin-35 mutants also show defects in the somatic gonad and intestine [4,5,8,13], both of which contribute to fertility [14,15]. Therefore, the tissue where expression of LIN-35 is most important for fertility remains unclear. Fertility is temperature sensitive across many animal species, including in C. elegans. The brood size of C. elegans peaks at temperatures around culture temperatures of 18-20˚C [16,17]. As culture temperatures increase from 20˚C to 26˚C, there is a reduction in brood size, but most hermaphrodites remain fertile [16,18]. However, above these temperatures (between 26˚C and 27˚C) virtually all hermaphrodites become sterile [18,19]. These phenotypes have been suggested to occur as a result of a decrease in functional sperm as temperature increases, followed by an additional loss of oocyte function at higher temperatures [17][18][19]. Interestingly, many lin-35 mutant phenotypes are also temperature sensitive, including the synthetic multivulva phenotype, larval growth (high temperature larval arrest phenotype), misregulation of developmental chromatin compaction, and ectopic germline gene expression [13,20,21]. In addition, the brood size of lin-35 mutants is decreased when grown at 25˚C compared to at 20˚C [5]. However, since wild-type fertility is also decreased across this temperature range, it is unclear if this represents a temperature sensitive phenotype or if lin-35 mutants are showing the same trend as wild type.
In this study, we show that lin-35 mutants display temperature sensitive fertility defects. Using somatically expressed LIN-35(+) transgenes to bypass the high temperature larval arrest phenotype, we uncover temperature sensitive decreases in brood size in lin-35 mutants that are greater than the reductions in brood size seen in wild type. During moderate temperature stress, lin-35 mutants also have defects in oocyte formation. Finally, we show that while lin-35 expression in the germ line is most critical for fertility, lin-35 expression in somatic tissues also plays an important role in maintaining fertility.

Temperature treatments
Three temperature treatments were used in this study. (1) Continuous exposure to 20˚C: experiments were performed on strains maintained continuously at 20˚C. (2) Continuous exposure to 26˚C: P0 hermaphrodites were up-shifted to 26˚C at the L4 stage and experiments were done on F1 hermaphrodites that had experienced their entire lifespan at 26˚C (3) Upshift from 20˚C to 26˚C: hermaphrodites were developed at 20˚C until the L4 larval stage and then up-shifted to 26˚C and experiments were done on the upshifted P0 worm.

Western blot analysis
Eighty F1 progeny raised at either 20˚C or 26˚C were collected for total protein samples in wild type, lin-35(n745), and strains containing somatic lin-35::GFP transgenes. Total protein from the entire sample was run on a 7.5% SDS-PAGE gel and transferred to a Nitrocellulose membrane. Membranes were blocked (5% non-fat dry milk in PBS) and incubated in 1:1000 Rabbit anti-LIN-35 [23] in block and 1:200 Mouse anti-TUBB-E7 (Developmental Studies Hybridoma Bank, University of Iowa, Iowa City, IA, USA) in block overnight at 4˚C. Membranes were blocked a second time and incubated with 1:2000 Goat anti-rabbit-HRP (Thermo-Fischer Scientific, USA) and 1:2000 Goat anti-mouse-HRP (ThermoFischer Scientific, USA) conjugated antibodies for one hour in the dark at room temperature. Blots were visualized via HRP-chemiluminesence (Amersham RPN2232) and exposure to X-ray film.

Brood size assay
To assay brood size, 10-15 L4 hermaphrodites were cloned out onto individual plates and placed at the assay temperature based on the temperature treatment protocol being used (see above). Each hermaphrodite assayed was moved to fresh plates daily until no embryos were seen on plates. F2 progeny were counted during the L4 larval stage or as young adults. Brood size assays of mated hermaphrodites were conducted following a similar protocol except that 3 N2 males raised at 20˚C were added to the plate. All hermaphrodites and males were transferred to seeded NGM plates daily, with the addition of three new N2 males raised at 20˚C on day 3 post-initial mating. Statistical analysis was performed with exclusion of sterile hermaphrodites using either two-way ANOVA or students T-test in GraphPad PRISM (GraphPad Software Inc., La Jolla, CA, USA).

Sperm localization assay
Sperm localization was observed by mating N2 males stained with MitoTracker Red CMXRos (Invitrogen) with mutant hermaphrodites. P0 mothers were plated at 20˚C or upshifted to 26˚C at the L4 larval stage. Males raised at 20˚C were stained with 50μL of 50μM Mitotracker Red CMXRos in M9 for 2-4 hours in the dark. 25 stained males were placed onto NGM plates seeded with HB101 E. coli containing approximately 20 F1 L4 hermaphrodite larvae (raised at 20˚C or 26˚C) and allowed to mate for 12-16 hours. Mated hermaphrodites were transferred to a clean plate for 1 hour before imaging to allow any recently deposited male sperm to localize within the germ line. Worms were imaged on a 2% agarose pad in 10mM Levamisole. Normarski and fluorescent images were acquired using a Nikon Eclipse TE2000-S (Nikon Instruments Inc., Elgin, IL, USA) inverted microscope at 60X with Q-Capture Pro (QImaging, Surrey, BC, Canada) imaging software. Sperm were scored as either primarily localizing within the spermatheca, a mixture of localization to the uterus and spermatheca, or primarily localized within the uterus.

Oocyte counting
Adult hermaphrodites were imaged live using Normarski optics and images were acquired using a Nikon Eclipse TE2000-S (Nikon Instruments Inc., Elgin, IL, USA) inverted microscope at 60X with Q-Capture Pro (QImaging, Surrey, BC, Canada) imaging software. One gonad arm was imaged per hermaphrodite and scored for the presence of one or more than one oocyte in the proximal gonad. Oocytes were defined as cells having a large clear nucleus and taking up the entire width of the proximal gonad.

Immunohistochemistry
For L1 staining: Young adult P0 mothers were placed in 1XM9 solution without bacteria and allowed to lay embryos overnight at 20˚C. F1 L1 larvae were transferred to slides coated with poly-L-Lysine, freeze cracked, and fixed with methanol/acetone [13]. For adult germ lines: Young adults were transferred without bacteria to slides coated with poly-L-Lysine in 0.1 mM levamisole. Germ lines were dissected before being freeze cracked, and fixed with methanol/ acetone. For both sample types: slides were blocked (1.5% BSA, 1.5% OVA, 0.05% NaN 3 in PBS) and incubated with primary antibodies overnight at 4˚C. For L1 animals 1:10,000 rabbit anti-PGL1 [24] was used. For adult germ lines DSHB-GFP-8H11 antibody at 1:100 concentration was used. DSHB-GFP-8H11 was deposited to the Developmental Studies Hybridoma Bank (DSHB) by the DSHB, created by the NICHD of the NIH and maintained at The University of Iowa, Department of Biology, Iowa City, IA 52242. Samples were incubated with secondary antibodies conjugated with Alexa Fluor 568 or 488 at 1:300 (ThermoFischer Scientific, USA) in PBS for 1 hour at room temperature. Slides were treated with 5mg/ml DAPI in 50 mL PBS for 10 min, washed 3 times with PBS for 10 min at room temperature and mounted on gelutol mounting medium. Z-stacks were taken using Nikon A1R Inverted Microscope Eclipse Ti confocal microscope with NIS Elements AR 3.22.09 at 60X.

Somatically expressed lin-35 can rescue high temperature arrest and allow for the analysis of germline temperature sensitivity
Germ line development and function occur primarily after L1 stage, which precludes the direct analysis of temperature influences on the adult germ line in lin-35 mutants because approximately 100% of these animals display high temperature larval arrest (HTA) at 26˚C [13]. The native lin-35 locus is ubiquitously expressed and previous experiments have shown that larval arrest and germline defects can be rescued using the endogenous promoter [22]. To determine if the germline defects observed in lin-35 mutants are temperature sensitive in a manner similar to somatic lin-35 phenotypes [13,20,21], we used two transgenic strains in which lin-35 is absent from the germ line, but animals could still bypass the larval arrest phenotype. First, we expressed lin-35::GFP from the elt-2 promoter (elt-2p::lin-35::GFP), which allows for intestinal specific expression of lin-35(+) (Fig 1A). We have previously shown that the elt-2p::lin-35::GFP transgene in a lin-35 mutant background can rescue the HTA phenotype, while leaving all non-intestinal tissues mutant for lin-35 function [13]. We also created a transgenic line that expressed lin-35::GFP in all somatic cells under the ubiquitously expressed let-858 promoter (let-858p::lin-35::GFP). These two lin-35(+) transgenes are maintained on extrachromosomal arrays, which are generally silenced in the germ line irrespective of the promoter used [25]. Indeed, our analysis of both of LIN-35(+)::GFP in both L1s and adults revealed no detectible expression in the germ line (S1 Fig) [13]. These two lin-35(+) transgenes allowed us to analyze the function of lin-35 in the germ line during moderate temperature stress.

lin-35 mutants display a temperature sensitive decrease in brood size that is only partially rescued by somatic expression of lin-35::GFP
To determine if lin-35 mutants demonstrate temperature sensitive germline defects, we scored both the brood size of individual worms and the percentage of fertile hermaphrodites in populations grown under two different moderate temperature stress treatments when lin-35 was missing from the germ line. lin-35 mutants have been previously shown to have a reduced brood size at 20˚C; however, the cause of the reduction is unknown [5]. We confirmed that lin-35(n745) mutants grown at 20˚C had a drastically reduced brood size; however, the percentage of fertile worms in the population was only weakly reduced compared to wild type ( Fig 1B and 1C). lin-35 mutants expressing either somatic lin-35(+) transgene at 20˚C showed a partial, but significant rescue in brood size compared to the lin-35(n745) mutant without a transgene ( Fig 1B). These data indicate that the loss of fertility in lin-35 mutants at 20˚C is partly due to loss of lin-35 function in somatic tissues.
As temperature increases to 26˚C brood size decreases in wild-type C. elegans hermaphrodites, but the percentage of hermaphrodites that are fertile remains close to 100% (Fig 1B and  1C) [18,19]. To allow for comparison between our transgenic lines and lin-35(n745) mutants with no lin-35(+) transgene, we grew hermaphrodites at 20˚C until the L4 stage when they were upshifted to 26˚C (upshift). These conditions allowed us to bypass the HTA phenotype seen in lin-35 mutants because the exposure to 26˚C occurred after most of larval development. The brood size in upshifted lin-35 mutants with or without a somatically expressed lin-35(+) transgene was significantly lower than wild type ( Fig 1B). However, unlike our findings at 20˚C, the lin-35(+) transgenes could not rescue the brood size compared to lin-35(n745) mutants without a lin-35(+) transgene ( Fig 1B). As the different genotypes displayed different brood sizes at 20˚C, we compared the relative brood size of each strain upon upshift to that same strain's brood size at 20˚C (Fig 1D). We found that lin-35 mutants with or without somatic lin-35(+) transgenes had a stronger relative decrease in brood size compared to the wild type when upshifted to 26˚C (Fig 1D). Similarly, the percentage of fertile worms in the population in upshifted worms was equivalently lower in lin-35 mutants with or without a somatic transgene compared to wild type ( Fig 1C). Overall, the fertility of lin-35 mutants was more sensitive to increases in temperature at the L4 stage than wild type, and expression of wild-type lin-35 in the soma did not rescue this temperature sensitivity. We next investigated the effects on fertility of raising hermaphrodites for their entire development at 26˚C. While lin-35 mutants expressing either somatic lin-35(+) transgene showed similar effects on brood size when animals were raised at 20˚C, whether upshifted or not, there was a marked difference in brood size between the lin-35(+) transgenes in lin- 35(n745) mutants raised at 26˚C. While lin-35 mutants that expressed the intestinal lin-35(+) transgene showed a significantly lower brood size than wild type at 26˚C, lin-35 mutants that expressed the pan-somatic lin-35(+) transgene had brood sizes that were indistinguishable from the wild type ( Fig 1B). Additionally, the relative decrease in brood size between worms raised at 20˚C and 26˚C was greater in lin-35 mutants that expressed the intestinal lin-35(+) transgene than those that expressed the pan-somatic lin-35(+) transgene (Fig 1D). In fact, at 26˚C lin-35 mutants expressing pan-somatic lin-35(+) displayed a similar reduction in brood size to wild type. Finally, the percentage of the population that was fertile was significantly lower in lin-35 mutants that expressed the intestinal lin-35(+) transgene than those that expressed the pansomatic lin-35(+) transgene (Fig 1C). Overall, these data underscore a clear difference in the capacity of pan-somatically expressed lin-35(+) to rescue fertility over intestinal-specific expression of lin-35(+) during moderate temperature stress.
The rescue of brood size at 26˚C in lin-35 mutants with the pan-somatic lin-35(+) transgene, but not the intestinal lin-35(+) transgene, suggested that lin-35(+) is important in nonintestinal somatic lineages when worms are raised at 26˚C. lin-35 has previously been shown to function in the somatic gonad [8]. Therefore, we expressed lin-35::GFP under the mir-786 promoter, which is expressed in the somatic gonad [26], in combination with intestinal expression of lin-35(+). However, no increase in brood size was observed during moderate temperature stress with expression of lin-35(+) in the somatic gonad ( Fig 1E). As even pan-somatic expression of lin-35(+) could not rescue fertility under all temperature treatments, there is likely a germline intrinsic function for LIN-35 that is necessary for preserving fertility during moderate temperature stress.
In the course of our experiments, we observed that pan-somatic expression of lin-35(+) in a wild-type background (i.e. somatic overexpression of lin-35(+)) resulted in a reduced brood size compared to wild type without the lin-35(+) transgene (Fig 1B). In a lin-35 mutant background, this lin-35(+) transgene fully rescues the HTA phenotype and rescues fertility close to the level of the lin-35(+) transgene in the wild-type background at 20˚C. One possible explanation for the reduced brood size when lin-35(+) is expressed in wild type is that the high levels of LIN-35::GFP driven by the strong let-858 promoter may display a dominant negative effect in somatic tissue that interacts with the germ line, which is supported by previous work that has shown that LIN-35 can bind its own promoter [22,23].

Germline expression of lin-35:GFP strongly rescues brood size in lin-35 mutants
To determine if LIN-35 has an intrinsic function in the germ line for preserving fertility at 26˚C, we analyzed lin-35 mutants with germline-specific expression of lin-35 using two different germline-specific promoters: pie-1p::lin-35::GFP and mex-5p::lin-35::GFP [22]. Because somatic expression of lin-35 is necessary to rescue the HTA phenotype, we could only examine the effects of temperature in upshifted L4 hermaphrodites. Expression of lin-35(+) from either transgene resulted in partial, but highly significant rescue of brood size at both 20˚C and upon upshift from 20˚C to 26˚C (Fig 2A). Additionally, the relative decrease in brood size between worms upshifted to 26˚C and worms raised at 20˚C for either germline-specific lin-35(+) transgene was equivalent to wild type (Fig 2B). This contrasts with soma-only expression of lin-35(+) that resulted in no rescue of brood size and a greater relative decrease in brood size when challenged with temperature upshift (Fig 1D). Therefore, germline intrinsic expression of LIN-35 is primarily responsible for preserving brood size, especially under moderate temperature stress.

Zygotic lin-35 expression is sufficient and necessary for fertility levels under moderate temperature stress
We next investigated whether the increased loss of fertility at 26˚C in lin-35 mutants was a maternal effect phenotype like the HTA [13] and synMuv phenotypes [9]. To achieve this, we tested the fertility of first generation lin-35(n745) homozygous M+Z-mutants from heterozygous mothers where lin-35(n745) is balanced by the hT2 translocation that expresses GFP in the pharynx (M+Z-, defined as an animal that has a wild-type lin-35 maternal load (M+) but is mutant for lin-35 zygotic expression (Z-)). The hT2 translocation itself causes a high penetrance of embryonic lethality in a wild-type background [27]. Therefore, we compared lin-35 (n745) M+Z-hermaphrodites to wild-type first generation GFP-progeny of +/hT2 hermaphrodites (Fig 3A). At 20˚C, lin-35(n745) M+Z-homozygotes displayed an intermediate brood size between that observed in wild type and lin-35(n745) M-Z-homozygous mutant hermaphrodites (M-lacking a maternal load) (Fig 3A). Thus, at 20˚C the decreased brood size in lin-35 mutants is partially maternal effect. However, at 26˚C lin-35(n745) M+Z-hermaphrodites were completely sterile, indicating that loss of just zygotic lin-35(+) expression is sufficient to result in a total loss of progeny production at 26˚C.
To further investigate the role of zygotically expressed lin-35 in fertility we crossed lin-35 (n745) M-Z-hermaphrodites with wild-type males that carried a GFP marker and scored the brood size of the resulting lin-35 heterozygote M-Z+ hermaphrodites. These worms lacked maternally loaded LIN-35(+) protein and mRNA in early embryogenesis, but expressed zygotic LIN-35(+) protein. At 20˚C lin-35(n745) M-Z+ hermaphrodites had a significantly smaller average brood size than wild-type hermaphrodites, but a significantly higher average brood size than lin-35(n745) M+Z-hermaphrodites (Fig 3A). At 26˚C lin-35(n745) M-Z+ hermaphrodites had an average brood size that was not significantly different than wild-type hermaphrodites ( Fig 3A). Thus, zygotic expression alone can strongly rescue the fertility of lin-35 mutants under non-stress temperatures and completely rescue fertility during moderate temperature stress.

Mating does not significantly rescue fertility in lin-35 mutants under most conditions
Decreased brood size in wild-type worms at 26˚C is primarily due to decreased sperm function [17][18][19]. To determine if the significant decrease in brood size in lin-35 mutants at 26˚C is also primarily due to decreased sperm function, we crossed lin-35(n745) hermaphrodites with and without lin-35(+) somatic transgenes to wild-type males raised at 20˚C to provide functional sperm. At 20˚C, the number of sperm is the limiting factor in brood size [28]; therefore, crossing with a male to provide more sperm generally leads to an increase in brood size. For hermaphrodites grown at 20˚C, there was a significant increase in brood size in all wild-type strains when mated. However, for strains with a lin-35(n745) mutation, only those expressing the pan-somatic lin-35(+) transgene showed a significant increase in brood size when mated (Fig 3B). The number of functional sperm made by hermaphrodites is reduced at 26˚C; therefore, crossing with a male raised at 20˚C should also increase brood size if oocyte function is  (Fig 3B). Thus, the reductions in brood size in lin-35 mutants are unlikely to be solely caused by dysfunctional sperm.

Pan-somatic expression of LIN-35(+) is necessary for proper sperm localization under moderate temperature stress
There are two possible scenarios that could lead to the observation of lin-35 mutants with or without somatic lin-35(+) transgene expression showing limited increases in brood size when mated. First, when lin-35 mutant hermaphrodites mate with males there could be limited sperm transfer/migration to the spermatheca. Second, lin-35 mutants could have oocyte defects that limit the ability of male sperm to generate cross progeny [29]. To test the first scenario, we labeled male sperm by incubating males with mitoTracker Red and then mated them with hermaphrodites for a period of 12-16 hours. We then assessed if sperm were present in hermaphrodites and able to migrate to the spermatheca (Fig 4A and 4B). Strong localization of male sperm to the spermatheca was observed in most strains for hermaphrodites raised at either 20˚C or 26˚C, despite some sperm remaining in the uterus (Fig 4A and 4B). The only animals without sperm localization to the spermatheca were lin-35(n745) mutants with intestinal expression of lin-35(+) grown at 26˚C. Overall, these data indicate that sperm can localize correctly in lin-35 mutant hermaphrodites raised at 20˚C, suggesting that oocyte defects may be present in lin-35 mutants even at 20˚C. However, as male sperm can still localize to the spermatheca at 26˚C when lin-35(+) is expressed broadly in the soma, it is likely that additional factors contribute to the low brood size of lin-35 mutants during moderate temperature stress.

Pan-somatic LIN-35(+) is necessary for oocyte formation under moderate temperature stress
To determine if the loss of fertility in lin-35 mutants could be due to an inability to produce oocytes, we quantified the number of oocytes in lin-35(n745) hermaphrodites with and without lin-35(+) transgenes at 20˚C and 26˚C. Each germ line was analyzed to determine if animals had more than one oocyte in a single gonad arm (Fig 5A). Close to 100% of gonads had >1 oocyte in all strains except for lin-35(n745) mutants expressing the intestinal lin-35(+) transgene at 26˚C (Fig 5B). Germ lines in lin-35(n745); elt-2p::lin-35::GFP hermaphrodites at 26˚C generally looked small, had poor organization, and typically contained only one cell proximal to the spermatheca that resembled an oocyte (Fig 5A). These data underscore the importance of broad somatic expression of lin-35(+) for maintaining germline function, especially under moderate temperature stress.

Discussion
In this study, we have shown that brood size in lin-35 mutants displays temperature sensitivity that goes beyond what is seen in wild type. We demonstrated that although LIN-35 has a small role in the soma to protect fertility, expression of LIN-35 in the germ line is of primary importance for fertility, especially under conditions of moderate temperature stress. Interestingly, rescued brood size at both 20˚C and with upshift. * significantly different than wild type at the same temperature, # significantly different than lin-35 mutants at the same temperature. P-value � 0.05 using two-way ANOVA with Turkey correction. Error bars indicate ± SEM. (B) The decrease in fertility with moderate temperature stress represented by the percentage of the mean brood size remaining under upshift conditions compared to growth at 20˚C within the same strain. With upshift, lin-35 mutants expressing germline lin-35(+) retain a similar relative brood size to wild type.
https://doi.org/10.1371/journal.pone.0286926.g002 Somatic expression of LIN-35(+) partially rescued brood size in lin-35 mutants grown from the L1 stage at either 20˚C or 26˚C. The generation of progeny relies on many somatic tissues, including intestines, somatic gonads, and neurons [14,15,31]. Given that lin-35 mutants display a broad range of effects on different tissues (including slowed growth), it is not surprising that somatic rescue of LIN-35(+) could have a positive effect on brood size. LIN-35 function solely in the intestine appears to be particularly important for brood size at 20˚C, but is not sufficient to overcome fertility deficits of lin-35 mutants at 26˚C. Marked differences in rescue between intestinally and pan-somatically expressed LIN-35(+) were seen with many phenotypes assessed, including brood size, percentage of fertile worms, sperm localization, and oocyte number. Although the somatic gonad is a necessary for fertility, expression of LIN-35 (+) in both the somatic gonad and intestine was not sufficient to rescue the low brood size seen in lin-35 mutants. The other somatic tissues in which in LIN-35(+) expression is required to maintain brood size during moderate temperature stress remain to be determined.
Pan-somatic expression of LIN-35(+) in lin-35 mutants rescued brood size in animals upshifted to 26˚C at the L1 stage, but did not restore brood size or the percentage of the fertile population when the upshift occurred at the L4 stage. What underlies these differences? There is other evidence in support of short and more acute temperature stresses at the L4/adult stage having stronger effects on germline biology. For example, short periods of growth at increased temperatures can increase asynapsis likely due to disruptions to the physical properties of the synaptonemal complex [32,33]. When C. elegans are kept at increased temperatures for longer periods they can acclimate to the conditions. Therefore, we propose that somatic expression of LIN-35(+) supports the correct development of somatic tissues during extended periods of developmental temperature stress to help maintain fertility. However, expression of LIN-35(+) in the soma is less effective at mitigating the effects of short and acute temperature stresses, which may more directly affect germline processes and therefore would require LIN-35(+) expression in the germ line to mitigate these effects.
Our somatic rescue experiments relied on multi-copy transgenic arrays, which can result in overexpression of transgenes. Therefore, we cannot rule out that overexpression of LIN-35(+) in the soma may contribute to the level of the rescue of fertility associated phenotypes observed. However, we have presented strong evidence that full fertility during moderate temperature stress is dependent on the expression of LIN-35(+) in somatic tissues outside of the intestine. First, pan-somatic expression of LIN-35(+) only weakly rescued the defects in brood 26˚C. M-Z-animals did not inherit any wild-type lin-35 products (protein or RNA) from their mother and did not have a wild-type copy of the lin-35 gene, M+Z-animals inherited lin-35 products from their mother but did not have a wild-type copy of the lin-35 gene, and M-Z+ did not inherit any wild-type lin-35 products (protein or RNA) from their mother but did have a wild-type copy of the lin-35 gene. * significantly different than wild type at the same temperature. P-value � 0.05 using one-way ANOVA with Turkey correction. Error bars indicate ± SEM. (B) Mean brood size of fertile worms for each genotype under the different mating and growth conditions (continual growth of hermaphrodites at 20˚C (blue) and continual growth at 26˚C (red)). Mating with wild-type males did not significantly increase the brood size of lin-35 mutants expressing either somatic transgene at high temperature. * significantly different from same strain unmated at the same temperature. P-value � 0.05 using students t-test. Error bars indicate ± SEM. For mated experiments, males were always grown to adulthood at 20˚C and then mating occurred at the temperature of the hermaphrodite treatment.
https://doi.org/10.1371/journal.pone.0286926.g003 size but strongly rescued other phenotypes, such as slow growth and high temperature larval arrest. This suggests that the level of somatic expression in a lin-35 mutant background was not detrimental to somatic development. Second, the transgenes used for germline rescue of fertility were low-copy number and genomically integrated [22]. As these transgenes could strongly rescue brood size under all temperature treatments, but not provide complete rescue, it is likely that somatic expression of LIN-35(+) is important for some fertility phenotypes. Therefore, expression of LIN-35(+) in somatic tissues is likely necessary to fully restore brood size in lin-35 mutants. A range of germline associated phenotypes and broad disruptions in germline gene expression are seen in lin-35 mutants [5,12,34], making it difficult to define specific roles for LIN-35 (+) in the germ line. The only well-defined role of LIN-35(+) the germ line is in its promotion of germline apoptosis. A slightly lower level of physiological apoptosis is seen in lin-35 mutants under non-stress conditions [6], but these animals fail to increase the rate of germline apoptosis under various stress conditions, including DNA damage and starvation [6,35]. Germline apoptosis is also induced under specific elevated temperature conditions [19]. Therefore, a decreased level of germline apoptosis may contribute to the reductions in fertility seen in lin-35 mutants, especially under stress conditions. Our data also reveals a role for LIN-35(+) in the formation and/or function of mature oocytes, which appeared to be especially sensitive to temperature when LIN-35(+) was only present in the intestine. Finally, increased fertility defects are present in double mutants between lin-35 and members of the MuvB core of the DREAM complex [34]. Thus, interactions between LIN-35 within the DREAM complex may be important for regulating gene expression in the germ line. Further research is needed to define the targets of a LIN-35/MuvB Core complex in the germ line and the impact temperature may have on the expression of these targets.
Many phenotypes associated with lin-35 mutants are notably temperature sensitive. For example, lin-35 mutant larvae only arrest at 26˚C, and increased temperature enhances the ectopic expression of germline expressed genes in somatic tissues and the synthetic multivulva phenotype seen in lin-35 mutants [13,20]. In this study, we have shown that both brood size and oocyte formation in lin-35 mutants are also temperature sensitive, expanding the temperature sensitive nature of lin-35 phenotypes to the germ line. Historically, temperature sensitive phenotypes are most clearly explained by a hypomorphic allele of a gene that produces a modestly changed protein, the folding of which is destabilized at increased temperatures. However, the lin-35(n745) allele contains a premature stop codon, has no visible protein expression, and has generally been categorized as a putative null allele [3]. Therefore, it seems unlikely that loss of functional LIN-35 protein is what ties these temperature sensitive phenotypes together. Another possible explanation could be that a protein complex in which LIN-35 normally interacts becomes unstable during moderate temperature stress when LIN-35 is absent. The most well characterized complexes involving LIN-35 are the E2F complex and the DREAM complex [34,36,37]. Previous work has shown that loss of other members of the E2F complex cause distinctly different germline phenotypes and lead to different changes in gene expression in mutant germ lines [12]. No studies to date have investigated the formation of the DREAM complex in the germ line. However, even at 20˚C in the soma there is a significant loss of the DREAM complex binding to its targets in lin-35 mutants, suggesting that phenotypes associated with loss of the complex are not temperature sensitive [34]. Increased loss of DREAM complex binding could occur at 26˚C, but this is yet to be explored. Therefore, the exact nature of the temperature sensitivity of germline phenotypes in lin-35 mutants remains unclear.
In summary, we have shown that fertility in animals lacking LIN-35 is highly sensitive to moderate temperature stress, which is primarily a zygotic effect. LIN-35 is required in the germ line for fertility, and broadly needed within somatic tissue for oocyte formation and/or function. Thus, we provide additional evidence for the central role that LIN-35 plays in protecting animals against conditions of stress.